NCERT class 12 Physics exemplar for Chapter 15 Communication Systems provides here are written by experts after extensive research on each and every topic to provide apt and authentic information to the students. If you go through previous year question papers you will get a clear picture of questions on exemplary book are directly asked in the examination. In order to score good marks, it is very important for the students to solve and get well versed with the exemplary questions and problems given in this exemplary book.
This NCERT class 12 Physics exemplar for Chapter 15 pdf present different varieties of questions like MCQ’S, fill in the blanks, match the following, true or false, short answer questions along with numerical problems, important formulas, exercises and assignments. This book helps you to understand the concepts clearly and help you to memorise the topic for a long period of time.
Class 12 Physics NCERT Solutions for Communication Systems
Communication systems are simply a collection of systems used for transmission, connection, communication and interconnection. These systems are categorically arranged into three different types on the basis of uses such as Media, Technology and Application area. Sensors, Transducers, Emitters and Amplifiers are all examples of modern technology which are used as components in the majority of modern devices.
Major examples of general communication systems:
 Internet
 Television
 Radio
 Computer
Concepts involved in chapter 15 Communication System are:

 Introduction
 Elements Of A Communication System
 Basic Terminology Used In Electronic Communication Systems
 Bandwidth Of Signals
 Bandwidth Of Transmission Medium
 Propagation Of Electromagnetic Waves
 Ground wave
 Skywaves
 Space wave
 Modulation And Its Necessity
 Size of the antenna or aerial
 Effective power radiated by an antenna Ex 15.7.3 – Mixing up of signals from different transmitters
 Amplitude Modulation
 Production Of Amplitude Modulated Wave
 Detection Of Amplitude Modulated Wave.
Communication systems are simply a collection of systems used for transmission, connection, communication and interconnection. These systems are categorically arranged into three different types on the basis of uses such as Media, Technology and Application area. Sensors, Transducers, Emitters and Amplifiers are all examples of modern technology which are used as components in the majority of modern devices.
Q.1: For beyond – the – horizon communication, which of the following frequencies will be suitable using sky waves?
(1) 10 kHz
(2) 10 MHz
(3) 1 GHz
(4) 1000 GHz
Soln:
(2) 10 MHz
The signal waves needs to travel a large distance for beyond – the – horizon communication.
Because of the antenna size the 10 kHz signals cannot be radiated efficiently.
The 1 GHz – 1000 GHz (high energy) signal waves penetrate the ionosphere.
The 10 MHz frequencies get reflected easily from the ionosphere. Therefore, for beyond – the – horizon communication signal waves of 10 MHz frequencies are suitable.
Q.2: Frequencies in the UHF range normally propagate by means of :
(1) Ground Waves
(2) Sky Waves
(3) Surface Waves
(4) Space Waves
Soln:
(4) Space Waves
Due to its high frequency, an ultra high frequency (UHF) wave can cannot travel along the trajectory of the ground also it cannot get reflected by the ionosphere. The ultra high frequency signals are propagated through line – of – sight communication, which is actually space wave propagation.
Q.3: Digital signals
(i) Do not provide a continuous set of values
(ii) Represent value as discrete steps
(iii) Can utilize binary system
(iv) Can utilize decimal as well as binary systems
State which statement(s) are true ?
(a) (1), (2) and (3)
(b) (1) and (2) only
(c) All statements are true
(d) (2) and (3) only
Soln:
(a) For transferring message signals the digital signals uses the binary (0 and 1) system. Such a system cannot utilise the decimal system. Discontinuous values are represented in digital signals.
Q.4: For line – of – sight communication, is it necessary for the receiving antenna to be at the same height as that of transmitting antenna? A TV transmitting antenna is at a height of 81 m. If the receiving antenna is at ground level, how much of the service area the transmitting antenna can cover?
Soln: In line – of – sight communication, between the transmitter and the receiver there is no physical obstruction. So, there is no need for the transmitting and receiving antenna to be at the same height.
Height of the antenna, h = 81m
Radius of earth, R = 6.4 x 10^{6}m
d = 2Rh, for range
The service area of the antenna is given by the relation :
A = nd^{2} = n(2Rh)
= 3.14 x 2 x 6.4 x 10^{6} x 81
= 3255.55 x 10^{6 }m^{2} = 3255.55 = 3256 km^{2}
Q.5: For transmitting a message signal a carrier wave of peak voltage 12v is used. In order to have a modulation index of 75% what should be the peak voltage of the modulating signal?
Soln:
Given:
Amplitude of carrier wave, A_{c} = 12v
Modulation index, m = 75% = 0.75
Amplitude of the modulating wave = A_{m}
Modulation index is given by the relation :
m = \( \frac{A_{m}}{A_{c}} \)
Therefore, A_{m} = m.A_{c}
= 0.75 x 12 = 9v
Q.6: A modulating signal is a square wave, as shown in the figure.
The carrier wave is given by
(1) Sketch the amplitude modulated waveform
(2) What is the modulation index.
Soln:
The amplitude of the modulating signal, A_{m} = 1v can be easily observed from the given modulating signal.
Carrier wave is given by, c(t) = 2 sin(8nt)
Amplitude of the carrier wave, A_{c} = 2v
Time period, T_{m} = 1s
The angular frequency of the modulating signal is given by,
\(\omega _{m} = \frac{2\pi }{T_{m}}\)= 2π rad s^{1} …(1)
The angular frequency of carrier signal, \(\omega _{c} = 8\pi \) rad s^{1} …(2)
from eqns.(1) and (2),
we get, \(\omega _{c} = 4\omega _{m} \)
The modulating signal having the amplitude modulated waveform is shown in the figure:
(2) Modulation index, m = \( \frac{A_{m}}{A_{c}} \) = \( \frac{1}{2} \) = 0.5
Q.7: For a wave having amplitude modulation, the minimum amplitude is found to be 2V and maximum aplitude is found to be 10V. Find the modulation index µ. If the minimum amplitude is 0V, what would be the value of µ?
Soln:
Given,
Max. Amplitude, A_{max }= 10v
Min. Amplitude, A_{min} = 2v
For a wave, modulation index µ, is given by :
µ = \( \frac{A_{max} – A_{min}}{A_{max} + A_{min}} \)
= \( \frac{10 – 2}{10 + 2} \) = \( \frac{8}{12} \) = 0.67
If A_{min} = 0,
Then,
µ = \( \frac{A_{max}}{ A_{min}} \) = 10/10 = 1
Q.8: During the transmission of AM wave, only the upper sideband is transmitted. But, at the receiving station, generation of carrier can be done. Show that if a device is available which can multiply two signals, then it is possible to recover the modulating signal at the receiver station.
Soln: Let, \(\omega _{c} \) be the carrier wave frequency
\(\omega _{s} \) be the signal wave frequencySignal received, V = V_{1} cos (\(\omega _{c} \) + \(\omega _{s} \))t
Instantaneous voltage of the carrier wave, V_{m} = V_{c} cos \(\omega _{c} \)t
V.V_{in} = V_{1}cos(\(\omega _{c} \) + \(\omega _{s} \))t. (V_{c} cos \(\omega _{c} \)t)
= V_{1}V_{c} [cos(\(\omega _{c} \) + \(\omega _{s} \))t . cos \(\omega _{c} \)t]
= \(\frac{V_{1} V_{c}}{ 2} \left [ cos{( \omega _{c} + \omega _{s}) t + \omega _{c}t} + cos{( \omega _{c} + \omega _{s})t – \omega _{c}t} \right ]\)
The low pass filter allows only the high frequency signals to pass through it. The low frequency signal \( \omega _{s} \) is obstructed by it.
Thus, at the receiving station, we can record the modulating signal, \( \frac{V_{1} V_{c}}{2}[\cos {(2\omega _{c} + \omega _{s})t + \cos \omega _{s} t}] \) which is the signal frequency.
 Transmitter, transmission channel and receiver are three basic units of a communication system.
 Low frequencies cannot be transmitted to long distances. Therefore, they are superimposed on a high frequency carrier signal by a process known as modulation.
 Two important forms of communication system are: Analog and Digital.
 Amplitude modulated waves can be produced by application of the message signal and the carrier wave to a nonlinear device, followed by a band pass filter.
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